There are essentially two groups of known similar techniques for performing chemical reactions in a fluidized bed.
A first group uses a dense fluidized bed [also called a "bubbling" bed] characterized by the existence of two zones having distinct particle concentrations within the reaction enclosure, with particle concentration being high in a first zone, e.g. 1,000 kg/m.sup.3 for a combustion fluidized bed, and much lower, less than 1 kg/m.sup.3 in a second zone above the first and separated therefrom by a relatively well-defined surface. The velocity difference between the gases and the solid particles is not large. In combustion reactors, combustion efficiencies are low, e.g. 85% to 95%, and the rates at which sulfur oxides and nitrogen oxides are rejected are significant, thereby limiting this technique to small plants.
Within this first group of techniques, a proposal has been made in patent document GB-A-No. 1 412 033 to split up a dense fluidized bed combustion reactor by means of an annular dividing wall, with the lower edge of the dividing wall being spaced apart from the fluidization grid, thereby obtaining a central dense bed region in which combustion takes place, and an annular dense bed region in which solid particles flow downwards, solely for the purpose of exchanging heat with a jacket surrounding the reactor. Some of the solid particles in the central dense bed region overflow the top of the annular dividing wall and move down in the annular dense bed region to return to the central combustion zone beneath the lower edge of the annular dividing wall. This type of apparatus suffers from the above-mentioned drawbacks of dense fluidized bed reactors, mainly the existence of a reaction zone having a very low concentration of particles above the dense bed. Further, it recirculates particulate matter taken solely from the upper part of the dense fluidized zone in the same manner as would a cyclone if located at the outlet from a circulating fluidized bed of the type described below.
A second group of known techniques makes use of a "circulating" fluidized bed of a type described in an article by REH published in the journal Chemical Engineering Progress, February 1971. This is described, in particular, in French Pat. Nos. 2 323 101 and 2 353 332 (Metallgesellschaft) [corresponding to U.S. Pat. No. 4,165,717]. It differs from the first group in particular by the lack of any separation surface between two zones and by the existence of a uniform reaction temperature throughout the reactor. The concentration or suspension density of particulate material varies substantially continuously from the bottom to the top of the reactor body, and the difference between the velocities of the gases and of the solid particles is much higher. For combustion reactors, combustion efficiencies are improved and the rates at which sulfur oxides and nitrogen oxides are rejected are lower. This technique is suitable for application to large plants, but it nevertheless suffers from drawbacks.
These may be observed, in particular, when the reaction is a combustion reaction. It is observed that the circulating fluidized bed reactor may be described as follows:
a) a higher zone of larger volume having a varying concentration of solid particles that is limited, but nevertheless sufficient. Heat exchange takes place in this higher zone, and in general it has tubes occupying the empty space in the reactor or it has walls lined with tubes through which a reactor cooling fluid passes. The concentration of particles varies from the bottom to the top of this zone, from about 50 kg/m.sup.3 to about 10 kg/m.sup.3, for example, and the figures may sometimes be even lower. In practice, the concentrations of particles provide heat exchange with the tubes lining the walls. The velocity of the gases is at full load is generally limited to values in the range 4 meters per second (m/s) to 6 m/s in order to avoid any risk of erosion; and
b) a lower zone of smaller volume having a much higher concentration of particles, varying from bottom to top, for example, from 500 kg/m.sup.3 to 50 kg/m.sup.3, i.e. in a ratio of 10:1, which ratio may even exceed 20:1 if the reactor is operating at half load, i.e. with the fluidization gas flowing at half velocity. This lower zone is where combustion takes place. A portion of the gas required for combustion, generally referred to as "primary" gas, is blown therein through a fluidization grid located at the bottom of the reactor. The major portion of the remainder of the combustion gas, called "secondary" gas, is injected at various levels above said grid, and the use of these levels may vary depending on the load in the reactor (with some of the levels being unused when the reactor is at partial load).
The velocity of the combustion flue gases in this lower zone of the reactor is determined by its varying section and by staged addition of secondary gas, and the desired velocity is practically the same as in the higher zone. This produces the large variation in concentration across the combustion zone which gives rise to several drawbacks:
incomplete combustion: concentration of unburnt particles and carbon monoxide may be high at the outlet of the reactor especially with some fuels that are difficult to burn;
desulfurization efficiency may be too low, thus requiring large quantities of desulfurization agent to be injected; and
limited flexibility in response to variations in reactor load, due to the requirement to have a minimum gas velocity for maintaining appropriate fluidization conditions, said velocity being about 3 m/s.
In order to improve temperature and combustion uniformity, it is therefore often necessary to increase the quantity of particulate material in the reactor, thereby increasing the amount of energy needed for fluidization.
In order to reduce these drawbacks, secondary gas injection at various levels is often used, and the ratio of secondary gas flow rate to primary gas flow rate is varied as a function of reactor load. However, this ratio can be varied only to a limited extent, since other criteria have to be taken into account, restricting flexibility:
burnout rate, which requires the primary gas flow rate to be maintained above a minimum value;
the requirement to maintain a reducing atmosphere in the lower part of the combustion zone of the reactor in order to keep the production of nitrogen oxides down to a minimum; and
the requirement to increase the excess of gas needed for combustion when the reactor load decreases in order to avoid excessively increasing the non-uniformity of particle concentration, while taking care to limit the production of nitrogen oxides as much as possible and to avoid significantly decreasing the thermal efficiency of the installation.
The wide range of said concentrations in the lower zone containing the combustion zone is thus a drawback and it would be advantageous to reach more uniform concentration between the various reactor levels which would not only improve combustion efficiency, but would also decrease fluidization energy consumption.
Unfortunately, a circulating fluidized bed cannot meet this requirement because of two special problems:
a) the velocity of the fluidzation gas in the combustion zone is related to the velocity selected in the higher zone where heat exchange occurs; and
b) solid particles are moving both up and down as shown in diagrammatic FIG. 1, and many solid particles of small size never move down to the vicinity of the fluidization grid, thereby producing vertical particle size stratification inside the reactor and causing the lower zone of the reactor to operate with larger sized particles. For example, the solid concentration in the first meter above the fluidization grid comes close to that of a dense bed, which is costly in energy and of no use for combustion.
Within this technique, other patents have provided various improvements to the operation of the circulating fluidized bed:
U.S. Pat. No. 4,594,967 and European patent No. 0 332 360 provide for the installation of a dense fluidized bed for capturing material at the outlet from the circulating fluidized bed, the material captured in this way reducing the amount of material captured in a conventional cyclone situated downstream from the expansion chamber constituting a clearance zone situated above the dense bed.
In these patents:
the dense fluidized bed is situated at the outlet from the circulating fluidized bed either on one side thereof U.S. Pat. No. 4,594,967 and patent EP 0 332 360 for a reactor that is rectangular in horizontal section), or else directly above it (EP patent No. 0 332 360 for a reactor that is circular in horizontal section);
the cyclone is situated either directly downstream from the expansion chamber (EP patent No. 0 332 360) or else after a tubular enclosure fitted with heat exchangers for reducing the temperature of the gases (U.S. Pat. No. 4,594,967) and therefore not forming a part of the circulating fluidized bed of the patent; in all of these cases the material taken by the dense fluidized bed reduces the material collected in the cyclone and does not alter the maximum quantity of material recycled in the circulating fludized bed; and
in both of those patents, the expansion chamber situated downstream from the zone containing the dense fluidized bed that does not have all of the fundamental characteristics of a circulating bed (uniform temperature, rising gas speed, solid matter concentration) that enable said zone to be used for transferring heat from the gas-solid mixture to the walls while retaining uniform temperature and gas-solid mixing good for containing chemical reactions.
Another patent, U.S. Pat. No. 4 788 919 provides for subdividing a single chamber into three, two, or one, with the boundaries between each of these chambers being provided by means of an expansion chamber whose section, as mentioned, may be four times that of the reactor, such that the gas velocity therein is no longer that of a circulating fluidized bed reactor. This velocity reduction makes it possible for material to be captured in the dense fluidized beds, thus greatly reducing the concentration of material in the or both the other upper chambers such that circulating fluidized bed operation takes place only in the bottom chamber, with the other two chambers and their extensions serving to capture additional small quantities of material and to provide additional cooling which leads the author of that patent to favor the design with one chamber only, thus reducing that patent to a disposition similar to that of Patents U.S. Pat. No. 4 594 967 and EP Pat. No. 0 332 360, i.e. to installing a dense fluidized bed at the outlet from a circulating fluidized bed reactor. In any event, in a design having a plurality of chambers, the same gas velocity is maintained therein.
In conclusion, these patents provide changes in the prior circulating fluidized bed type technique whose drawbacks are specified above.
Compared with said technique they are characterized by a reduction in the amount of solid material captured by the cyclones or separators, but they do not alter the pressure and concentration profile characteristics of the prior "circulating fluidized bed" technique.